This invention relates to a.c. power measurement in general and more specifically to an apparatus for measuring power by measuring a.c. currents accurately over a wide dynamic range of applied currents.
Known power measurement devices have a relatively limited dynamic range, on the order of 40 to 60 dB. Electric utility companies are not able to account for all of their generated power in a power distribution system at least partially because of the apparent losses attributable to inaccuracy in metering. The dynamic range limitation of conventional power meters means that it is not possible to measure power accurately under both high and low power drain conditions with the same device. There is thus a need to overcome inaccuracies in metering to improve the effectiveness of the electric power distribution system.
Power is the vector product of current and voltage. The dynamic range of the voltage in an electric utility system is generally narrowly limited so that power measurement accuracy thus hinges on the ability to measure a wide range of currents applied to a load.
Power measurement technology has developed three main approaches to measuring current: current transformers, shunts and Hall effect and like devices. Each approach has its limitations. Conventional current transformers exhibit a very limited dynamic range, since they saturate at high currents and lose sensitivity because of limited initial permeability at low currents. Current transformers also tend to saturate with small d.c. current flow caused by half-wave rectified loads, and they exhibit non-linear response because of the magnetizing current which causes amplitude and phase shift errors of the measured currents. Since instantaneous power is the product of instantaneous voltage and instantaneous current, any phase shifts can cause errors.
Shunts, i.e., resistive shunt measuring devices, also tend to have a very narrow dynamic range. Although measured voltage drop is proportional to current, heating is proportional to the square of the current. Hence, shunts tend to waste power and can overheat to the point of destruction in a wide dynamic range environment. Another restriction is that a shunt measuring circuit must be at the same potential as the shunt. This restriction makes it awkward to measure two simultaneous currents, as for example in 120/240 volt circuits where each is at a different potential.
Other electronic sensors, such as Hall effect devices tend to exhibit marked temperature sensitivity and provide limited long-term stability. This again is a limitation for many applications. Again, the measuring devices must be maintained at the same potential as the circuit to be measured, which limits their usefulness in electric utility system applications.
It is known that reducing the terminating resistance of a current transformer reduces the deleterious effects caused by magnetizing current, since the power load or burden seen by the transformer is reduced as load impedance is reduced. However, as the resistance of a conventional current transformer is reduced, the voltage output (which one desires to measure) is also reduced, so that a compromise is required in practice between the desire to obtain sufficient signal to overcome background noise and the desire to provide as low a "burden resistance" to the current transformer as possible in order to minimize the deleterious effects of phase and amplitude distortion in a current transformer circuit. In practice, the burden cannot be set to zero because the secondary winding resistance is an integral part of the burden.
The following patents were reviewed in the course of an investigation of the patent literature with respect to the present invention:
Milkovic U.S. Pat. No. 4,492,919 describes a three-path low impedance current sensor with an active load for measuring high amplitude currents. The feature emphasized is a meander leg forming the shunt, the shunt itself sharing common input and output nodes with the current legs. Also disclosed is an active circuit for sensing current, but the active circuit fails to take into account the effects of secondary resistance and thermal imbalance have upon operation of a meter over a wide dynamic range.
Wolf et al. U.S. Pat. No. 4,240,059 describes a recent shunt-type current divider for sensing current through a flat disk or sheet wherein the shunt paths are transverse of the main current legs and of different length. Significantly, the shunt is formed integrally with the legs. The invention in this patent is not suitable for applications employing a prewound toroidal core mounted on the shunt. Nor does it effectively balance out hum pickup. The integral structure renders it impossible to mount a closed toroid on the shunt. The shunt is not circular in cross section and could not be manufactured to be so in the disclosed embodiment.
Other patents of interest were also uncovered. Some of these patents were references to the foregoing patents.
McCormack, U.S. Pat. No. 2,818,544 is an early patent which describes the concept of zero impedance load circuits. However, the structures disclosed therein fail to show or suggest structures of the type described in connection with the present invention.
Johnson, U.S. Pat. No. 2,831,164 describes a toroidal transformer apparatus. It teaches a type of current divider to control the effective ratio of a current transformer with a toroidal core.
Bradstock et al. U.S. Pat. No. 2,915,707 describes a current measuring reactor arrangement for measuring current in a bus bar. Specifically, this patent discloses a three path arrangement wherein all current flows between nodes common to all three legs. The tow low impedance legs are equal in length and enclose dual toroidal cores on the central conductor. However, a primary teaching of this patent is the use of a dividing current shunt which balances out the field to reduce hum pickup.
Wolf et al. U.S. Pat. No. 4,182,982 brought some of the early concepts up to date with the combined use of two separate secondary windings and an electronic amplifier.
U.S. Pat. No. 3,372,334 to Fenoglio et al. describes still another current shunt arrangement. In particular this patent describes a dividing shunt.
Friedl U.S. Pat. No. 4,513,273 describes a specific structure for a current transformer and differential current shunt. It teaches about the geometry of differential resistors. It also employs an active element (an amplifier) having a first secondary winding as a sensor and driving a second secondary winding in series with a load.
De Vries U.S. Pat. No. 4,580,095 describes a specific structure for a current divider. This patent is representative of a class of current dividers which would be considered unsuited to use with the present invention.
Friedl U.S. Pat. No. 4,626,778 describes an active current sensor and structure. The disclosure is similar to that of the '273 patent.
Halder U.S. Pat. No. 4,628,251 describes a voltage transducer in connection with an active circuit. The current transformer employs multiple windings. The active circuit employs an active impedance transformer, specifically a voltage buffer, to drive an operational amplifier. Nothing seems to suggest attention to correction of the problem of secondary winding resistance in the context of current measurement.
Willis, U.S. Pat. No. 1,084,721, describes an early design for a shunt used in a measuring instrument.
Lienhard, U.S. Pat. No. Re. 31,613, describes various embodiments of measuring transformers and cores.
Lienhard, U.S. Pat. No. 4,506,214, describes various embodiments of measuring transformers and cores.
All of these references describe apparatus applying approaches distinguishable from the present invention in the context of the desire to measure a.c. current over a wide dynamic range. A power meter is nevertheless needed, and more particularly, a current measuring device is needed which is capable of accurate measurement of current over a wide dynamic range, on the order of 100 dB.